1. Field of the Invention
This invention relates to AM stereophonic transmitting systems for the transmission of at least two signals on a single carrier and, more particularly, to such improved systems for transmitting fully compatible AM stereophonic signals.
2. Description of the Prior Art
Interest in transmitting stereophonic information over the AM frequency band has existed for more than 50 years, nearly as long as commercial AM broadcasting, itself, has existed. During this time, many different schemes have been suggested for communicating the stereophonically-related audio signals from the broadcasting station to the radio receivers. None of these schemes, however, has met with general approval by the broadcasting community since none has demonstrated a clear superiority over the others.
A number of criteria is commonly used in comparing the performance of the various systems. Generally stated, these criteria include the quality of stereophonic reproduction in stereophonic receivers and the compatibility of the transmitted stereo signal for reception by currently available (monaural) AM receivers. In addition, it is desired that the stereophonic signals transmitted should not occupy any greater RF bandwidth than that presently allocated for monaural AM transmission.
More specifically, the stereophonic performance of an acceptable AM stereo system should be such that, upon reception, the signal-to-noise ratio is as great as possible. In any event, it should not be significantly degraded as compared to reception obtainable with current monaural systems. Also, the distortion introduced by the transmission and reception of the stereo signal should be minimal. Finally, the separation between the stereophonically related signals, usually referred to as the left signal (L) and the right signal (R), should be as great as possible.
With respect to monaural compatibility, any acceptable AM stereo system must be fully compatible with monaural receivers currently available on the market. In other words, the detection of the composite stereo signal with the monaural envelope detectors currently in use should produce a signal corresponding to the sum (L+R) of the two stereophonically related signals, without noticeable distortion. Additionally, the loss in the loudness of the received signal in monaural receivers due to the stereophonic nature of the broadcast signal should be as low as possible.
An AM stereophonic broadcast system, typical of the prior art, includes a transmitter station and a receiver station. The transmitter station illustratively includes two signal sources. For the broadcast of audio signals, the two signal sources provide the left signal (L) and the right signal (R). Such systems encode the left signal (L) and right signal (R) to form two new component signals (L+R) and (L-R). One of these component signals is applied to an amplitude modulator to modulate a carrier wave; the other component is used to phase (or frequency) modulate the carrier wave.
The receiving station receives the transmitted signal and supplies it to an RF front end for detection, amplification and conversion to an intermediate frequency IF signal. The IF signal is applied to a stereophonic decoder or demodulator to reconstruct the left and right source signals (L) and (R). The reconstructed left and right signals (L) and (R) are amplified and applied to respective speakers to reproduce the stereophonic sound.
One such stereophonic broadcast system is described in U.S. Pat. No. 4,218,586 of Parker et al. The Parker et al. broadcast system matrixes the left and right signals (L) and (R) to provide a first component signal (1+L+R) and a second component signal (L-R). The (L+R) component signal directly amplitude modulates the transmitted signal. The (L-R) component signal phase modulates the transmitted signal in a manner simulating a quadrature modulated signal. FIGS. 1A, 1B and 1C are vector diagrams illustrating the relationship of these modulation component signals. In these figures, the unmodulated carrier wave C is taken as a reference for both amplitude and phase. In FIG. 1A where the left signal (L) has been set equal to the right signal (R) as would occur for monaural program material, the (L+R) component signal adds to and subtracts from the carrier amplitude resulting in pure amplitude modulation; the (L-R) component signal is zero under this condition and the resultant vector I is always in phase with the carrier signal C. In FIG. 1B where the left signal (L) has been set equal to the negative of the right signal (R) to permit only quadrature modulation, the (L-R) component signal is added in quadrature with the carrier signal C producing a resultant wave (E), whose phase leads or lags the carrier signal C as the (L-R) component signal varies over its positive and negative excursions.
The instantaneous phase angle is labeled .phi. in these figures. FIG. 1C shows the general case in which the left signal (L) and right signal (R) have no special relationship. In FIG. 1C, both amplitude and phase modulation result. Normalizing to the carrier signal C, i.e., C=1 .angle..phi., the instantaneous amplitude of the resultant wave (E) is expressed as: EQU E={(1+L+R).sup.2 +(L-R).sup.2 }.sup.1/2 ( 1)
and its instantaneous phase is expressed as: ##EQU1##
In the Parker et al. broadcast system, the first encoded component (1+L+R) is applied to a first amplitude modulator to produce the signal as shown in FIG. 1A, and the second encoded component is applied to a second amplitude modulator. An RF exciter provides a first carrier signal to the first modulator and to a phase shifter, which applies a second carrier signal phase shifted by 90.degree. with respect to the first carrier signal to the second modulator. The outputs of the first and second modulators are summed to provide a signal as shown in FIG. 1C. This signal may be represented mathematically as: EQU ECos(Wt+.phi.), (3)
where E is defined by equation (1) above. Parker recognized that a monaural receiver using a conventional envelope detector would detect the envelope portion or E of this signal in accordance with equation (3) and would produce an undistorted signal only when the left signal (L) equals the right signal (R). Parker et al. proposed to make his broadcasted signal compatible with normal monaural receivers by removing the amplitude portion E of the signal in accordance with equation (3) by a limiter, leaving only the phase portion. The resulting phase portion is amplitude modulated according to Parker et al. by a signal component (1+L+R) in a high level modulator. The transmitted signal may be represented by (1+L+R)Cos (wt+.phi.), which is the equivalent of the output from the adder multipled by Cos .phi., where Cos .phi. equals: ##EQU2##
The transmitted signal of Parker et al. is compatible when it is received by a monaural receiver incorporating an envelope detector. An envelope detector is oblivious of the phase component of the transmitted signal and will demodulate the transmitted signal to produce the component signal (1+L+R).
A receiver can decode the transmitted stereophonic signals by using a synchronous detector operating in phase with respect to the carrier signal C to demodulate the (1+L+R) component signal and a quadrature synchronous detector to demodulate the (L-R) component signal. Summing these component signals produces a left signal (L) and subtracting produces a right signal (R). The "1" term is a DC component which can be removed by capacitive coupling. Parker et al. disclose a stereophonic receiver, wherein the received signal is limited and, then, compared by a multiplier with the phase of the carrier signal Cos wt, which is locked to the phase of the RF exciter in the transmitter. The output of the multiplier Cos .phi. is applied to a corrector circuit along with the received signal, whereby a signal in the form of equation (3) is reproduced. The output of the corrector circuit is applied to a first multiplier acting as a synchronous detector, where it is multiplied by Cos .phi. and is shifted positively by 45.degree., and to a second multiplier acting as a synchronous detector, where it is multiplied by Cos .phi. and shifted negatively by 45.degree.. The outputs of the first and second multipliers correspond respectively to the component signals (1+L+R) and (L-R). These component signals are in turn applied to a conventional matrix decoder, which provides the left signal (L) and right signal (R).
The most complex part of the Parker et al. stereophonic modulation procedure is the generation of the phase modulation .phi.. It is the purpose of this invention to disclose an improved method and apparatus for generating this phase modulation component. As discussed above, Parker et al. employs a limiter to produce their phase modulation component, which is in turn applied as a carrier source for the standard AM transmitter. There are inherent difficulties in its precise implementation. First, the production of full stereophonic signal at radio frequency requires two modulation steps which must be in accurate magnitude and phase relationships. Second, a severe requirement is placed on the limiter used to remove the amplitude component. Limiting must be performed over a very wide dynamic range. Very careful design and adjustment of the limiter circuitry is required to minimize incidental phase shifts.